netdev - Re: [PATCH bpf-next v7 11/13] net-timestamp: add a new callback in tcp_tx

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Message-ID: <67a24989d7202_bb56629425@willemb.c.googlers.com.notmuch>
Date: Tue, 04 Feb 2025 12:08:25 -0500
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Jason Xing <kerneljasonxing@...il.com>, 
 Martin KaFai Lau <martin.lau@...ux.dev>
Cc: davem@...emloft.net, 
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Subject: Re: [PATCH bpf-next v7 11/13] net-timestamp: add a new callback in
 tcp_tx_timestamp()

Jason Xing wrote:
> On Tue, Feb 4, 2025 at 9:16 AM Martin KaFai Lau <martin.lau@...ux.dev> wrote:
> >
> > On 1/28/25 12:46 AM, Jason Xing wrote:
> > > Introduce the callback to correlate tcp_sendmsg timestamp with other
> > > points, like SND/SW/ACK. We can let bpf trace the beginning of
> > > tcp_sendmsg_locked() and fetch the socket addr, so that in
> >
> > Instead of "fetch the socket addr...", should be "store the sendmsg timestamp at
> > the bpf_sk_storage ...".
> 
> I will revise it. Thanks.
> 
> >
> > > tcp_tx_timestamp() we can correlate the tskey with the socket addr.
> >
> >
> > > It is accurate since they are under the protect of socket lock.
> > > More details can be found in the selftest.
> >
> > The selftest uses the bpf_sk_storage to store the sendmsg timestamp at
> > fentry/tcp_sendmsg_locked and retrieves it back at tcp_tx_timestamp (i.e.
> > BPF_SOCK_OPS_TS_SND_CB added in this patch).
> >
> > >
> > > Signed-off-by: Jason Xing <kerneljasonxing@...il.com>
> > > ---
> > >   include/uapi/linux/bpf.h       | 7 +++++++
> > >   net/ipv4/tcp.c                 | 1 +
> > >   tools/include/uapi/linux/bpf.h | 7 +++++++
> > >   3 files changed, 15 insertions(+)
> > >
> > > diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> > > index 800122a8abe5..accb3b314fff 100644
> > > --- a/include/uapi/linux/bpf.h
> > > +++ b/include/uapi/linux/bpf.h
> > > @@ -7052,6 +7052,13 @@ enum {
> > >                                        * when SK_BPF_CB_TX_TIMESTAMPING
> > >                                        * feature is on.
> > >                                        */
> > > +     BPF_SOCK_OPS_TS_SND_CB,         /* Called when every sendmsg syscall
> > > +                                      * is triggered. For TCP, it stays
> > > +                                      * in the last send process to
> > > +                                      * correlate with tcp_sendmsg timestamp
> > > +                                      * with other timestamping callbacks,
> > > +                                      * like SND/SW/ACK.
> >
> > Do you have a chance to look at how this will work at UDP?
> 
> Sure, I feel like it could not be useful for UDP. Well, things get
> strange because I did write a long paragraph about this thing which
> apparently disappeared...
> 
> I manage to find what I wrote:
>     For UDP type, BPF_SOCK_OPS_TS_SND_CB may be not suitable because
>     there are two sending process, 1) lockless path, 2) lock path, which
>     should be handled carefully later. For the former, even though it's
>     unlikely multiple threads access the socket to call sendmsg at the
>     same time, I think we'd better not correlate it like what we do to the
>     TCP case because of the lack of sock lock protection. Considering SND_CB is
>     uapi flag, I think we don't need to forcely add the 'TCP_' prefix in
>     case we need to use it someday.
> 
>     And one more thing is I'd like to use the v5[1] method in the next round
>     to introduce a new tskey_bpf which is good for UDP type. The new field
>     will not conflict with the tskey in shared info which is generated
>     by sk->sk_tskey in __ip_append_data(). It hardly works if both features
>     (so_timestamping and its bpf extension) exists at the same time. Users
>     could get confused because sometimes they fetch the tskey from skb,
>     sometimes they don't, especially when we have cmsg feature to turn it on/
>     off per sendmsg. A standalone tskey for bpf extension will be needed.
>     With this tskey_bpf, we can easily correlate the timestamp in sendmsg
>     syscall with other tx points(SND/SW/ACK...).
> 
>     [1]: https://lore.kernel.org/all/20250112113748.73504-14-kerneljasonxing@gmail.com/
> 
>     If possible, we can leave this question until the UDP support series
>     shows up. I will figure out a better solution :)
> 
> In conclusion, it probably won't be used by the UDP type. It's uAPI
> flag so I consider the compatibility reason.

I don't think this is acceptable. We should aim for an API that can
easily be used across protocols, like SO_TIMESTAMPING. Taking a
timestamp at sendmsg entry is a useful property for all such
protocols.